The invention relates to a fluid driven pump suitable for use in a reverse osmosis or filtration system for example a water purifying system, and to a system employing such a pump.
The preferred embodiment of the current invention provides a high pressure fluid output flow from two or more lower pressure fluid input flows. It is specifically but not exclusively intended to provide high pressure water for purification by reverse osmosis, where water contaminated by unwanted solutes is forced, at a pressure of typically 60 bar, through a semi-permeable membrane, effectively filtering out those solutes. This process is typically used for desalination of seawater.
It is a characteristic of reverse osmosis that the flow of contaminated water across the membrane needs to be typically ten times greater than the flow of purified water through the membrane, the excess flow acting to flush away contaminants accumulated at the membrane surface. In the simplest systems, this high pressure flushing flow is released to waste, taking with it, and wasting, typically 90% of the energy used to provide the high pressure input flow.
There are established methods to recover the energy contained in the high pressure flushing flow . . .
It may be used to drive a hydraulic motor mechanically coupled to the pressurising pump, with the balance of energy input provided by, for example, an electric motor. This method is complex and consequently expensive, with inevitable inefficiencies in the indirect mechanical transfer of energy. Standard, relatively inexpensive, hydraulic motors cannot be used since they are designed for use with hydraulic oils, relying on them for lubrication. PA1 It may be used more directly to provide part of the energy used by the pressurising pump. U.S. Pat. No. Re.32,144 and U.S. Pat. No. Re.33,135 describe mechanically driven reciprocating-piston pumps where one side of the piston and cylinder act as a pump and the other side acts as a motor, driven by the flushing flow to provide most of the pumping power. PA1 It recovers the energy otherwise wasted in the high pressure flushing flow, increasing system efficiency typically by a factor of ten. PA1 It has minimal mechanical losses, since the driving pressure is applied as directly as possible to provide pumping effort. PA1 It offers great simplicity, and consequent low manufacturing cost, since mechanical drives and linkages are eliminated.
A further characteristic of reverse osmosis is that the contaminated water input flow needs to be thoroughly filtered, to prevent clogging of the membrane by particulates. It is often cost-effective to provide an additional low-pressure pump to drive the contaminated water input flow through the filter, since this allows a higher pressure drop across the filter, typically 1 bar, enabling a smaller and cheaper filter to be used. Without this additional pump, the pressure drop across the filter would limited to around 0.5 bar by the poor ability of the high pressure pump to suck against low pressures. The additional pump will also be required if, as is often the case, the high pressure pump is not self-priming.
The preferred embodiment of the invention is directed to avoiding or mitigating at least some of the disadvantages of these known devices.